1. Field of the Invention
The invention relates to a method of winding optical fiber on a bobbin and, more particularly, to a method of winding optical fiber on a bobbin which permits free streaming of the fiber from the bobbin without clumping or knotting and which minimizes the effect on the optical signal passing through the fiber.
2. Description of Related Art
A number of weapons and communications systems have been developed or are under development which use an optical fiber for two-way data communication between two or more moving bodies or between a moving body and a fixed station. Examples of such uses include communication links between aircraft, between an aircraft and a ship, and between a projectile, such as a missile or mortar shell, and a control station at its launch site. Use of optical fiber for such communication precludes electromagnetic interference and compromising interception.
Optical fiber, however, has certain disadvantages not present in other forms of communication. Optical fiber is fragile rendering it subject to breakage while a wire communication system is stronger. Aside from breakage, optical fiber communication performance may be degraded by microcracks or microbends in the fiber generated by bending or other stresses imposed on the fiber. Such damage to an optical fiber not only reduces the fiber's long-term durability, but also causes losses in optical signal strength and content.
A typical optical fiber application involves packaging a continuous length of optical fiber on a bobbin inside a vehicle with one end of the fiber being attached to operational devices in the vehicle and attaching the other end of the fiber to a control or communication station at the launch site. After launching the vehicle, the optical fiber streams from the bobbin inside the vehicle allowing two-way communication with the vehicle during its flight.
The problem is to provide a reliable and compact means for packaging the optical fiber on the bobbin in a manner which minimizes stresses on the fiber, to preclude adverse effects on communication performance, and which permits reliable streaming deployment of the fiber during flight of the vehicle.
Conventional methods of winding optical fiber onto a bobbin involve translating and rotating the bobbin while applying the fiber. At the end of a fiber layer the translation direction is reversed while the winding direction is held the same, and the next layer applied. This results in the pitch between layers being reversed. On each turn the fiber crosses two turns in the layer below it as shown schematically in FIG. 1 which shows a conventional bobbin 5 with adjacent fiber layers 6,7. The upper layer 7 crosses over lower layer 6 at locations 8 on each side of the bobbin, only one side being visible in FIG. 1.
When optical fiber is wound by this method, small radius bends called microbends are induced at each crossover. The compressive stress in the glass optical waveguide at the microbends is a major contributor to optical attenuation in a wound bobbin. Successive layers of fiber increase the compressive stress in each microbend and increase optical attenuation proportionally. The length of fiber, and the range of the missile, are limited by the optical attenuation in the fiber between the optical transmitter and receiver, thus limiting the range between stations.
The point at which the layer being wound crosses over the layer below it is a function of winding tension, winding angle and surface friction of the fiber. The winding tension can be controlled within close tolerances, but the other two factors are not easily controlled. In order to sense the winding angle, complex and costly electronics are required. The winding process must be adjusted to account for changes in winding angle. The change in winding angle is, in effect, a change in the winding pitch, so that as the pitch changes the spacing between adjacent fibers is not uniform. This non-uniform spacing is cumulative and after several layers are wound, gaps tend to develop in the wind. These gaps cause the fiber turns in a layer to intermingle with the turns in the layer below and result in poor reliability when the fiber is stripped off the bobbin during transport between stations. This intermingling of fiber turns is called "slump".
In order for crossover to occur uniformly, the winding angle must change and the surface friction of the fiber must be overcome. Since the surface friction is a result of the manufacturing process, the friction coefficient changes with each manufactured batch of fiber. It is difficult to detect this change during the winding process. All these factors combine to make the winding process complex. This complexity leads to making the process labor intensive and reduces reliability. The major contributor to this complexity is the necessity to crossover the layer below.
Attempts have been made to provide a bobbin wound with multiple layers of optical fiber without having crossover fibers. In the method disclosed in U.S. Pat. No. 3,586,563 to Fukami et al., the winding direction is reversed following the completion of each separate layer causing the fiber to fold back on itself and form a loop adjacent the edge of the layer. While this method serves to reduce optical attenuation caused by crossover fibers, the freestanding loops cause "snagging" when the fiber is stripped from the bobbin at a high rate of speed during use. The free standing loops generally lie in plane perpendicular to the bobbin's surface and extend radially a height greater than the layer thickness as a consequence of the necessity for a non-zero loop radius. The loops thus create asperities in the wound fiber surface which can catch or "snag" the fiber from an overlying layer as it is stripped off.